The output wavelength of a Free Electron Laser (FEL) is determined by the electron beam energy and the wiggler or undulator parameters of wavelength and field strength. It may be desirable in certain applications to vary the wavelength of the FEL output. Although this could in principle be done by changing the wiggler parameters, such variation would occur too slowly for many applications.
Although the electron beam energy out of the accelerator can be changed more rapidly if done in the primary acceleration system, the physics of the electron beam transport from the accelerator to the wiggler may be affected in an undesirable manner which degrades or prevents the lasing process.
Accordingly, it would be desirable to provide a method for varying the wavelength of the output of an FEL in a manner that does not degrade or prevent the lasing process.